Progressive lenses work in a very simple way, it’s really actually progressive…therefore the lenses progressively change, that is, with as smooth transition as possible.
Let’s first take a look at what a prescription for progressive glasses would look like:
In either of these we can see where there is a number indicated for the “ADD” which stands for addition and refers to the reading power that is needed for these correction lenses.
On a progressive lenses the reading power is blended into the lenses. From the top to mid-way would be the distance correction, at mid way it would be the mid-distance or computer reading correction and at the bottom of the lenses the near reading (books, labels, etc)
The sketches below show give you a good idea.